1. Technical Field
The present disclosure relates to a terminal, a base station, a transmission method and a reception method.
2. Description of the Related Art
In 3rd Generation Partnership Project Long Term Evolution (3GPP LTE), orthogonal frequency division multiple access (OFDMA) is used as a downlink communication method, while single carrier-FDMA (SC-FDMA) is used as an uplink communication method.
In a downlink of 3GPP LTE, a synchronization channel (SCH), a physical broadcast channel (PBCH), a physical downlink control channel (PDCCH), a physical downlink shared channel (PDSCH), a physical multicast channel (PMCH), a physical control format indicator channel (PCFICH), and a physical hybrid automatic repeat request indicator channel (PHICH) are used. In an uplink of 3GPP LTE, a physical uplink control channel (PUCCH), a physical uplink shared channel (PUSCH), and physical random access channel (PRACH) are used.
A terminal (also referred to as user equipment (UE)) first catches SCH to establish synchronization with a base station (also referred to as eNB). Thereafter, the terminal reads BCH information (broadcast information) to acquire a parameter specific to the base station (see, for example, 3GPP TS 36.211 V11.5.0, “Physical channels and modulation (Release 11),” December 2013; 3GPP TS 36.212 V11.4.0, “Multiplexing and channel coding (Release 11),” December 2013; and 3GPP TS 36.213 V11.5.0, “Physical layer procedures (Release 11),” December 2013). After acquiring the parameter specific to the base station, the terminal sends a connection request to the base station via PRACH to establish communication with the base station. To the terminal for which communication has been established, the base station transmits, as required, control information via a control channel such as PDCCH or the like. The control information transmitted here from the base station includes information on resources allocated to the terminal by the base station, modulation and channel coding scheme (MCS) information, hybrid automatic repeat request (HARQ) process information, transmission power control information, terminal ID information, and the like.
In LTE, control signals and data signals are transmitted in units of subframes.
In LTE, HARQ is applied to downlink data from the base station to the terminal, and, in response to a downlink data signal, an acknowledgement (ACK) or a negative acknowledgement (NACK) is transmitted using an uplink channel such as PUCCH or the like. ACK or NACK (also referred to ACK/NACK signal or a response signal) is, for example, 1-bit information indicating ACK (no error) or NACK (error).
Multiple response signals transmitted from multiple terminals are spread, in a time domain, using a zero auto-correlation (ZAC) sequence having a zero auto-correlation characteristic, a Walsh sequence, and a discrete Fourier transform (DFT) sequence, as illustrated in FIG. 1, and are code-multiplexed in PUCCH. In FIG. 1, (W(0), W(1), W(2), W(3)) denotes a Walsh sequence with a sequence length of 4, and (F(0), F(1), F(2)) denotes a DFT sequence with a sequence length of 3.
As shown in FIG. 1, in the terminal, the ACK/NACK signal is first primarily spread in a frequency domain, using a ZAC sequence (with a sequence length of 12) into frequency components corresponding to 1SC-FDMA symbol. That is, the ZAC sequence with a sequence length of 12 is multiplied by the ACK/NACK signal components represented by complex numbers. Next, the primarily-spread ACK/NACK signal and the ZAC sequence functioning as a reference signal are respectively secondarily spread using a Walsh sequence (with a sequence length of 4: W(0) to W(3)) and a DFT sequence (with a sequence length of 3: F(0) to F(2)). That is, respective components of a signal with a sequence length of 12 (the primarily spread ACK/NACK signal or the ZAC sequence functioning as the reference signal) are multiplied by respective components of an orthogonal sequence (a Walsh sequence or a DFT sequence). Furthermore, the secondarily-spread signal is subjected to an inverse discrete Fourier transform (IDFT) or an inverse Fast Fourier transform (IFFT) thereby being converted into a signal with a sequence length of 12 in the time domain. Thereafter, a cyclic prefix (CP) is added to each signal having subjected to IFFT, and thus a 1-slot signal including 7 SC-FDMA symbols is formed.
PUCCH is located at each edge of a system band in the frequency domain. A resource for PUCCH is allocated, on a subframe-by-subframe basis, to each terminal. 1 subframe includes 2 slots, and PUCCH has frequency hopping (inter-slot frequency hopping) between a first slot and a second slot.
ACK/NACK signals from different terminals are spread using ZAC sequences defined by different cyclic shift indexes and orthogonal sequences corresponding to different orthogonal cover indexes (OC indexes). The orthogonal sequence is a combination of a Walsh sequence and a DFT sequence. Note that the orthogonal sequence is also called a block-wise spreading code sequence. Thus, by using despreading and correlation processing, the base station is capable of demultiplexes these code-multiplexed ACK/NACK signals (see, for example, Seigo Nakao, Tomofumi Takata, Daichi Imamura, and Katsuhiko Hiramatsu, “Performance enhancement of E-UTRA uplink control channel in fast fading environments,” Proceeding of 2009 IEEE 69th Vehicular Technology Conference (VTC2009-Spring), April 2009). FIG. 2 illustrates PUCCH resources defined by orthogonal cover indexes (OC index of 0 to 2) of an orthogonal sequence and cyclic shift indexes (0 to 11) of a ZAC sequence. In the case where a Walsh sequence with a sequence length of 4 and a DFT sequence with a sequence length of 3 are used, up to 3*12=36 PUCCH resources can be defined in the same time-frequency resource. However, all 36 PUCCH resources are not necessarily usable.
By the way, in recent years, it has been expected to realize, as a mechanism for supporting a future information society, machine-to-machine (M2M) communication to provide autonomous communication service between devices without a user intervening. A specific example of an application of an M2M system is a smart grid. The smart grid is an infrastructure system that efficiently supplies a lifeline such as electric power, gas, or the like. In the smart grid, M2M communication is performed between a smart meter installed in each family house or building and a central server to adjust a resource supply-demand balance in an autonomous and effective manner. Other examples of applications of the M2M communication system include a monitoring system for use in commodity management, telemedicine, or the like, remote management of a vending machine in terms of inventory or charging.
In the M2M communication system, an attention is on use of a cellular system that provides a particularly wide communication area. In 3GPP, in standardization of LTE and LTE-Advanced, M2M based on the cellular network is being discussed under the name of Machine Type Communication (MTC). In particular, further expanding the communication area is under discussion to handle a case where an MTC communication device such as a smart meter is installed at a location such as a basement of a building or the like that is not converted by the existing communication area (see, for example, 3GPP TR 36.888 V12.0.0, “Study on provision of low-cost Machine-Type Communications (MTC) User Equipments (UEs) based on LTE,” June 20135). For example, to achieve a further expansion of the communication area, it is under discussion to perform a repetition communication in which the same signal is repetitively transmitted multiple times.